The present invention relates to supported catalysts made up of modified supports and an active component, their production and their use in the hydrogenation of unsaturated compounds.
Supported catalysts, in particular supported oxides, play an important role in catalysis. A study of adsorbed O2 indicates that oxides supported on diamagnetic support materials are sometimes enriched in less electron-rich oxygen species such as O2xe2x88x92 and Oxe2x88x92 because their surface has only a low density of electron-donating centers. Since selective oxidation of organic molecules requires that only a limited amount of active O2 is made available, numerous mixed and supported oxides are employed here. Since their surfaces also have various adsorption sites, the modification of supports by application of further components (hereinafter referred to as modifiers) enables activity and selectivity of the catalysts to be influenced over wide ranges.
The support materials can interact with the active metal. Such interactions of the metals with reducible oxides resulting from high reduction temperatures (over 700 K) are generally referred to as strong metal-support interactions (SMSI). A description of SMSI assumes that ions of the support oxide migrate to the surface of the reduced metal at high reduction temperatures. Such modifiers enable the activity of the catalyst in various reactions to be increased. The increased activity is generally attributed to the active component being highly dispersed. In addition, the catalysts have a high sintering and agglomeration stability.
Carturan, G. Chim. Int. (Milan) 65 (1983) 688, describes the hydrogenation of polyunsaturated C18-fatty acids using palladium catalysts. One of the catalysts used comprises a support of glass beads which are coated with TiO2. The TiO2 is applied from an ethanolic Ti(OEt)4 solution, followed by heating to 400xc2x0 C. The modification of the support material is carried out here by pretreating the shaped body, namely the glass beads, with a precursor compound of the modifier.
Ryndin et al., Applied Catalysis, 63 (1990) 37 to 50, describe a study of the influence of group IVB ions on the adsorption and the catalytic properties as well as the dispersion of the active metal in the system Pd/SiO2. The catalyst system is produced from dehydrated SiO2 and tetrabenzyltitanium, tetrabenzylzirconium or tetrabenzylhafnium and subsequent adsorption of allyl(cyclopentadienyl)palladium. During production of the catalyst, air is excluded completely. The synthesis of MeOH from CO and H2 and the hydrogenation of benzene in the presence of the catalyst are studied.
EP-A 0 225 953 discloses catalysts comprising a metal alkoxide-modified support and a catalytically active metal of transition group VIII of the Periodic Table of the Elements. In the examples, the only support material used is xcex3-Al2O3 which has been modified with a metal alkoxide. The application of the catalytically active metals is carried out exclusively by impregnation with a solution of their chlorides or nitrates. The catalysts display increased activity in the disproportionation of alkenes, in a gas-phase process for preparing xcex1-substituted acrylate esters or methyl methacrylates, in processes for the selective hydrogenation of hydrocarbons having at least one alkene and one alkyne unit, where the alkyne is hydrogenated without hydrogenation of the alkene, and in a process for preparing methane from CO and H2.
K. Matsuo et al. Appl. Surf., pages 33, 34 (1998), 269-276, describe catalysts comprising a titanium-coated silica support in which metals of the Pt group are present as active component. The Ti/SiO2 support can be produced by various methods:
a) Gas-phase hydrogenation of titanium alkoxides and adsorption on SiO2;
b) Pyrolysis of titanium alkoxides and adsorption on SiO2;
c) Adsorption of titanium colloids on SiO2.
These supports are impregnated with metals of the Pt group, in the case of palladium with PdCl2. The metal-support interactions (SMSI) are examined in the total oxidation of acetone. In these studies, it was found that only the catalyst produced by method a) is active. This makes it clear that the properties and the catalytic activity of the modified supported catalysts are influenced by the method by which they are produced.
It is an object of the present invention to provide a catalyst and a process for producing this catalyst, which catalyst has the active component highly dispersed, has a high sintering and agglomeration stability and is superior to the catalysts described in the prior art, especially for use in the hydrogenation of unsaturated compounds.
We have found that this object is achieved by a supported catalyst comprising a catalytically active metal and a support modified by application of modifiers to an inner support material. The catalyst of the invention is produced by applying the catalytically active metal in the form of its chloride-free dissolved amine complexes, acetylacetonate complexes or allyl complexes to the modified support.
In the supported catalysts of the present invention, the catalytically active metal is dispersed particularly finely and in a particularly stable fashion. Avoiding the use of chloride ions prevents adverse effects of the chloride ions on the dispersion and activity of the catalytically active metal.
For the purposes of the present invention, modifiers are components which are applied to supports and influence the activity and selectivity of the catalyst.
For the purposes of the present invention, an inner support material is the support component still to be modified. The support is composed of inner support material and modifier.
The inner support material is preferably pretreated thermally prior to application of the modifiers. The modification of the inner support material by the modifier is based on the hypothesis that a hydrolysis-sensitive modifier compound (the modifier precursor) added in excess to the inner support material reacts with the surface hydroxyl groups of the inner support material. The number of surface groups available for this can be controlled, according to E. F. Vansant, P. Van der Voort and K. C. Vrancken xe2x80x9cStudies in Surface Science and Catalysisxe2x80x9d, Vol. 93, Chapter 4, pages 79 to 88, Editors: B. Delmon and J. T. Yates, Elsevier, 1995 and Ralph K. Iler xe2x80x9cThe Chemistry of Silicaxe2x80x9d, pages 624 to 637, John Wiley and Sons, 1979, for the example of SiO2, by thermal pretreatment of the inner support material. The thermal pretreatment of the inner support material accordingly has a decisive influence on the properties of the catalyst.
The inner support material used is preferably a material selected from the group consisting of Al2O3, SiO2, TiO2, ZrO2, aluminosilicates, MgO and activated carbon which has been chemically modified by surface oxidation. Particular preference is given to using SiO2 as inner support material.
The support material can be present in any form. Preference is given to powder, granules, extrudates, spheres, pellets or rings.
As modifier precursor, use is made of compounds comprising a metal of transition group IV, V, VI or VIII of the Periodic Table of the Elements. The modifier preferably comprises an element selected from the group consisting of Ti, Zr, Hf, V, Nb, Cr, Mo, W, Mn, Ta. Particular preference is given to using compounds of Ti, Zr, Nb, Ta as modifier.
The future modifiers are generally applied in the form (the precursor form) of their alkoxides, halides, oxalates or carboxylates. They are preferably applied in the form of their alkoxides. These should be hydrolysis-sensitive so that they can be converted into their oxides. Compounds selected from the group consisting of titanium tetraisopropoxide, titanium tetrachloride, zirconium bromide, zirconium isopropoxide, niobium(V) ethoxide, niobium(V) isopropoxide, niobium oxalate, niobium ammonium oxalate, tantalum chloride and tantalum ethoxide are particularly preferably used. Very particular preference is given to using titanium tetraisopropoxide and niobium(V) isopropoxide.
The modifiers interact with a catalytically active metal applied to the catalyst support, thus giving catalysts having an increased activity since the dispersion of the active species is increased compared to catalysts without modifiers. Furthermore, such modified supported catalysts have a better ageing stability owing to their increased agglomeration and sintering resistance.
As catalytically active metals, use is made of metals of groups I, VII and VIII of the Periodic Table of the Elements, preferably Co, Rh, Ni, Pd, Pt, Re, Cu or Ag. Particular preference is given to using Ni, Co, Pd, Pt and very particular preference is given to using Pd and Pt. Pd is most preferred.
The catalytically active metals are applied in the form of their chloride-free amine complexes, acetylacetonate complexes or allyl complexes, preferably in the form of their amine complexes. The counterions used are generally nitrate or hydroxide. Pd is particularly preferably applied in the form of [Pd(NH3)4](NO3)2 and Pt is particularly preferably applied in the form of [Pt(NH3)4](NO3)2.
The catalytically active metal can be distributed over the shaped body in any way. It is preferably present as a coating (shell) having a thickness of up to 2 mm, particularly preferably a thickness of up to 1 mm.
The present invention further provides a process for producing a catalyst according to the present invention, which comprises the following steps:
a) thermal pretreatment of the inner support material,
b) impregnation of the thermally pretreated inner support material with a modifier in the gas or liquid phase,
c) if desired, hydrolysis of the modified support material at from 20 to 200xc2x0 C. in an H2O-containing gas stream,
d) if desired, drying at from 100 to 500xc2x0 C.,
e) if desired, calcination at from 200 to 1000xc2x0 C.,
f) application of the catalytically active metal by impregnation or ion exchange,
g) drying at from 20 to 500xc2x0 C.,
h) if desired, calcination at up to 1000xc2x0 C.
The thermal pretreatment of the inner support material (support oxide) enables the number of available surface hydroxyl groups to be controlled. These react with the modifier precursor. The thermal pretreatment can be carried out in air or in a stream of nitrogen, generally at from 200 to 1200xc2x0 C., preferably from 400 to 800xc2x0 C. For example, the inner support material is, in the case of SiO2 granules, pretreated at from 720 to 870 K for from 6 to 8 hours prior to loading with the modifier precursor.
Essentially two routes can be distinguished for applying the modifier precursor to the inner support material. These are, on the one hand, methods of coprecipitation and cocrystallization of the various components (route 1), often via the formation of a gel as intermediate. This results in materials which have a bulk composition which is very similar to the composition of the surface. In contrast, it is also possible to apply substances to the surface of existing support materials (route 2) by, for example, exploiting adsorption properties, ion exchange capability or the possibility of forming chemical bonds between the support material and the component applied. The milling of solid pure materials and subsequent thermal treatment, which sometimes make solid state reactions between the components possible, can be considered as intermediate routes between the two principal methods distinguished above.
Preference is given to using the second route in the process of the present invention for producing the supported catalysts of the present invention. For this purpose, an impregnation of the inner support material is carried out in the liquid phase in a suitable solvent or in the gas phase.
Impregnation in the gas phase can be carried out, for example, by chemical vapor deposition (CVD). A suitable modifier precursor is, for example, TiCl4 which can be reacted with oxidic surfaces from the gas phase.
Impregnation in the liquid phase can be carried out from an aqueous or organic phase, depending on the modifier precursors used. The preferred metal alkoxide-modified supports are produced by impregnation of the desired inner support material with a solution of an alkoxide precursor of the desired metal oxide. Here, the solution used for the impregnation is preferably organic. The only proviso is that an adequate amount of the modifier precursor of the metal oxide chosen is soluble in the solvent. For the present purposes, an adequate amount generally means from 1 mol of modifier metal per 0.1-100 kg of inner support material, preferably per 1-50 kg of inner support material, particularly preferably per 5-25 kg of inner support material. Use is usually made of hydrocarbons or alcohol solvents, preferably tetrahydrofuran. The modified support material is, if desired, hydrolyzed in an H2O-containing gas stream at from 20 to 200xc2x0 C., preferably from 20 to 100xc2x0 C. This stream is, for example, an H2O-containing inert gas such as N2 or Ar or H2O-containing air or H2O-containing oxygen. Subsequently, the modified support material is, if desired, dried at from 80 to 500xc2x0 C., preferably from 100 to 200xc2x0 C., and, if desired, calcined at from 200 to 1000xc2x0 C., preferably from 400 to 800xc2x0 C. The calcination step is preferably omitted.
The resulting proportion of modifier oxide, based on the inner support material, is generally from 0.1 to 10% by weight, preferably from 0.5 to 5% by weight, particularly preferably from 1 to 3% by weight. If the modified support material is not hydrolyzed or not dried or not calcined, the modifier precursor can also be the desired target product of the first step.
The application of the catalytically active metal can be carried out by methods known from the literature, preferably by impregnation with the active components in the liquid phase, i.e. preferably by impregnation or ion-exchange processes. A review of the use of these two methods is given by Doorling et al. (T. A. Doorling, B. W. Lynch, L. Moss, J. Catal. 20 (1971), 190) for the example of the system Pt/SiO2. Important parameters in the application of the noble metals (active components) which influence the future dispersion of the active component are the pH of the aqueous impregnation solutions and the nature of the precursor of the active component. Sulfur-containing ions present can likewise have effects on the dispersion of the metal. Depending on the precursor compound used to introduce the catalytically active metal (active component), the impregnation can be carried out in aqueous or organic solution. If the catalytically active metal used is palladium, as is particularly preferred in the catalysts of the present invention, ion-exchange processes are preferably carried out using Pd(NH3)4(NO3)2 in ammoniacal, aqueous solution and impregnation processes are preferably carried out using diallylpalladium or palladium acetylacetonate in organic solution. As organic solvent, preference is here given to using alcohols or THF. Particular preference is given to using chloride-free palladium precursor compounds.
Subsequent to the impregnation, the supported catalyst obtained is dried at from 20 to 500xc2x0 C., preferably from 50 to 300xc2x0 C., particularly preferably from 100 to 250xc2x0 C., and, if desired, calcined at up to 1000xc2x0 C. The calcination step is preferably omitted.
The novel supported catalysts produced in this way generally contain from 0.01 to 1.5% by weight of Pd, preferably from 0.1 to 1.4% by weight of Pd, based on the total mass of the supported catalyst.
The dispersion of the Pd, measured by a method described by Th. Mang, Prxc3xa4paration, Charakterisierung und Aktivitxc3xa4t von Palladiumkatalysatoren mit Konzentrationsprofil, thesis, Munich, 1996, by pulse chemisorption of CO in a stream of hydrogen, is dependent on the application of the catalytically active metal (impregnation/ion-exchange processes) and the precursor compound used. The dispersion of Pd (reported in %) on a modified support is usually higher than that on an unmodified support, as the examples below for the system SiO2xe2x80x94TiO2/SiO2 show. In the catalysts produced by ion exchange or impregnation of TiO2-modified SiO2 supports using Pd(NH3)4(NO3)2 solutions, Pd dispersions of  greater than 50% can be achieved.
An increased dispersion of the catalytically active metal on modified supports compared to unmodified supports is retained even after a number of redox cycles, which makes it clear that the stability of the catalytically active metal particles is increased by the modification of the support.
The present invention further provides for the use of the catalyst of the present invention in a process for the hydrogenation of unsaturated compounds and also provides a process for the hydrogenation of unsaturated compounds in which a catalyst according to the present invention is used.
Examples of unsaturated compounds which can be used in such a process are acetylenes, dienes, olefins, aromatic systems, aldehydes, ketones, for example xcex1,xcex2-unsaturated aldehydes and ketones, and compounds bearing further functional groups, and also combinations thereof. Furthermore, the catalyst of the present invention can be used in a process for removing O2 or H2.
The hydrogenation can be carried out in the gas phase, in the liquid phase or in a mixed gas/liquid phase. The process can be carried out continuously or batchwise.
The process is carried out using known, literature methods for the hydrogenation of unsaturated compounds with the aid of supported catalysts. Reviews of the industrial hydrogenation of unsaturated compounds may be found in xe2x80x9cStudies in Surface Science and Catalysisxe2x80x9d, Vol. 27, Editor: L. ervený, Chapter 18 from M. L. Derrien, p. 613 to 665 and xe2x80x9cUllman""s Encyclopedia of Industrial Chemistryxe2x80x9d, Editors: B. Elvers, S. Hawkins, M. Ravenscroft and G. Schulz, 5th edition, Vol. A13, p. 487 to 497.
The process is generally carried out in a pressure range from 0 to 300 bar, preferably from 0 to 100 bar, particularly preferably from 0 to 50 bar, and in a temperature range from 0 to 400xc2x0 C., preferably from 0 to 200xc2x0 C., particularly preferably from 0 to 150xc2x0 C.